The focus position or the position of the focus plane is an important parameter for a high-energy beam used in workpiece processing. The location of the focus plane or the location of the focus position in the expansion direction of the high-energy beam depends on the divergence or the convergence of the high-energy beam in front of the focusing element as well as on the focal length of the focusing element for the high-energy radiation (or more precisely, for the wavelength range of the high-energy radiation, or the working wavelength if using a laser beam) used.
The divergence or convergence of the high-energy beam can normally be controlled well and is subject to little change following a basic adjustment. For great beam power, however, and in particular for CO2 laser radiation, the focal length of the focusing element may change during workpiece processing. For example, if a lens is used as the focusing element, a radial temperature gradient will form due to the inhomogeneous warming and cooling of the lens, which will lead to a change in the refractive index n of the lens, since n is dependent on the radial coordinate (i.e., n=n(r)). The refractive index change during workpiece processing can lead to a shortening of the focal length (so-called thermal lens). The actual focal length fFL of the focusing lens is therefore, among other things, a function of the beam power PL and the beam shape (for example the beam diameter DR), the degree of soiling of the focusing lens and the time t: fL=f(PL, DR, t).
Using a suitable measuring device, measurement of the beam path or of the beam caustics or recording the beam power in the vicinity of the beam waist can be performed to determine the focus position of a high-energy beam in the beam direction. It is also possible to cut the workpiece at several different selected distances between the focusing element and the workpiece, and to determine the focus position based on the width of the cut to the workpiece. The foregoing options do not typically allow recording the focus position during processing of the workpiece.